Welcome to the forum! Black powder is kind of in short supply right now, and even at the best of times I think it would be rare to find it for sale anywhere in California. In Wisconsin I gather that there is a dealer about a hundred miles to the South of my home, but it is in an area I do not like to visit. When I order black powder, I generally order ten pounds or more to get better use of the hazmat fee I pay. There are several online dealers who sell it. Check your local laws before ordering. Most online dealers know where not to ship powder, but some do not.... and you could get into a real pickle if you receive a box of explosives at your residence without jumping through the proper legal hoops.
Oh for the days when you could just pick some up at the hardware store along with Dynamite and anything else you might need around the farm!
Welcome from Lancaster county, PA.
Rural here. The local hardware dropped dynamite ages ago. But they still have black powder. Keep it stored in what looks like a brick outhouse 20 yards from the building.